Undersea communication systems are undergoing a continuing evolution to keep pace with the ever increasing demands made on them. Most share the use of an acoustic carrier or a related modulated scheme thereof for telemetry or bidirectional communications.
At first, marine engineers and scientists tried to adapt proven radio communication techniques to undersea applications and have met with some success. A whole family of related radio frequency communication systems developed which modulated one or both sidebands and transmitted them separately or along with a carrier. Synchronous or asynchronous demodulation at the radio receivers was performed in along predictable, expected manners so that the wide variety of radio communication techniques were acceptable for a good many applications.
The routine radio approaches that were adapted by undersea communications designers used acoustic means instead of radio frequency means for carrying the modulating information. Single sideband and redundant sidebands were acoustically modulated but the separate modulation of the transmitted upper sideband and the separate modulation of the lower sideband have not been adapted to allow an increased communication capability in the undersea environment.
Thus, there is a continuing need in the state of the art for an acoustic technique for the discrete modulation of the upper and the lower sidebands of an acoustic carrier frequency that allows for an asynchronous as well as a synchronous modulation and demodulation capability and which also provides for an ultrasonic shifting of two discrete information sources to assure their separation for transmission and reception through a water medium.